Conventional winches may be installed on a ceiling, and may be used to move some kind of electric device, for example a light source, such that an electric device connected to the cable hangs down from the ceiling with a predetermined distance from the ceiling. In the prior art, such a winch is driven by an electric motor, and is controlled by switching electric power on for moving the winch, and switching electric power off as soon as the object hangs down from the ceiling at the desired distance. The velocity by which the object is moved up and down is given by the speed of an electric motor in the winch at a voltage which is provided to the winch. However, in a winch hanging an electric device from the ceiling by a cable, the cable carrying the object is usually wound around a cable roll to move the electric device up or down. Taking into account that the diameter of the cable which is wound-up around the cable roll decreases as more cable is unwound from the roll, the linear velocity by which the electric device is moved downward also depends on the amount of cable which is coiled on the cable roll, with the effect that the velocity by which the object is moved downward by a conventional winch is not constant even if the cable roll of the winch is driven with a constant voltage, resulting in a constant number of rotations per time.
Hence, the motion of an electric device connected to the cable of a conventional winch cannot be controlled precisely with respect to velocity and/or acceleration of the connected electric device. This is a particular disadvantage in the case where a plurality of electric devices, for example, lamps, have to be moved simultaneously with a particular relationship between their locations so as to form some kind of predetermined spatial pattern.
Therefore, there is a need for a winch which offers a better control of the motion of an electric device connected to its cable while the cable is unwound up to a predetermined length.